Pressurized-water nuclear reactors have a core consisting of prism-shaped fuel assemblies arranged vertically and resting on a support plate within the vessel of the nuclear reactor.
During the operation of the nuclear reactor, flux measurements have to be carried out periodically inside the core itself. For this purpose, fission detectors of very small size are used, and these are moved by remote control, by means of teleflex cables, inside tubes closed at one of their ends, called glove fingers. The glove fingers are introduced according to a predetermined distribution over the entire height of some of the fuel assemblies of the core, after passing within instrumentation tubes. By moving the flux detectors within the glove fingers introduced into the fuel assemblies, flux measurements can be made over the entire height of the core. It must be possible to extract the glove fingers from the fuel assemblies of the core, for example in order to make it possible to conduct the operations of refuelling the reactor core; for this purpose, a pull is exerted on the end of the glove fingers from an instrumentation room arranged laterally relative to the reactor vessel well. The glove fingers are arranged in guide tubes, one of the ends of which opens into the instrumentation room and the other end of which opens into the inner volume of the vessel by way of a vertical sleeve leading through the vessel bottom, this vessel bottom being of rounded shape. The guide tube has at least one bent portion between its two ends.
The lower internal equipment of the reactor comprises, in addition to the core support plate on which the fuel assemblies rest, the shroud and partitioning of the core and a set of elements arranged between the core support plate and the lower rounded bottom of the vessel, and comprising, in particular, instrumentation guide columns, each delimiting a central guide conduit in the extension of the inner bore of the corresponding lead-through sleeve. The end of each of the lead-through sleeves is engaged in the inlet end of the corresponding guide conduit with a relatively large radial play which makes it easier to install the lower internal equipment and match each of the lead-through sleeves to the corresponding guide conduit.
The inside diameter of the tubes, guide conduits and fuel-assembly instrumentation tubes is such that sufficient play remains between the glove finger and its guide conduit. The glove fingers can therefore be maneuvered easily by pulling and pushing, in order respectively to extract them from the core over the entire height of the latter, i.e., over a length of about four meters and in order to reintroduce them into the core.
However, it was found that, after a certain reactor operating time, the force required to extract the glove fingers and, above all, the force needed to reintroduce them increased substantially. For instance, the insertion force of the glove fingers which, for example, can be 100 to 150 Newtons at the time of commissioning of a reactor can, in this case, increase to a value of 400 to 500 Newtons after the second refuelling. This increase in the insertion force of the glove fingers was found to be attributable to the presence of solid particles settling between the glove finger and the inner wall of the guide tube, especially near the bends of this guide tube. In fact, the glove fingers, during their movement, cause an accumulation of particles in the region of the bends and assist their consolidation.
A process for cleaning the guide tubes at the time of the operations to refuel the reactor core was therefore proposed. At this time, the vessel is open in its upper part and is in communication with the reactor pool, the assembly as a whole being filled with water. To carry out the cleaning process, means were provided for injecting demineralized water under pressure into the guide tubes at their end opening into the instrumentation room.
However, this demineralized water and the particles which it contains in suspension have to be recovered at the other end of the guide tubes, in order to prevent radioactive particles from falling to the bottom of the water-filled vessel.
It is therefore necessary to recover the demineralized water at the outlet of the lead-through sleeves. To gain access to these sleeves, it is customary to dismantle the lower internal equipment which is extracted from the vessel, the upper internal equipment and the core assemblies having been extracted previously.
Means for recovering the water and the radioactive particles are fitted in succession on the end of each of the sleeves during the injection of pressurized water into the corresponding guide tube.
These means are installed from the platform of the fuelling machine above the reactor pool by means of a pole of very great length which, in its lower part, carries a camera making it possible to display the operations of installing the recovery means. These means comprise a cylindrical cap equipped on the inside with a gasket of a diameter corresponding to the diameter of the sleeve, a flexible hose which communicates with the inner volume of the cylindrical cap and on which is arranged a valve controlled from the platform by means of a control linkage, and a filtration unit at which the flexible hose terminates above the level of the pool.
These operations are difficult to carry out, and it is not possible to monitor them under very good conditions by means of a camera fixed to the lower end of the pole.
On the other hand, in order to carry out this process, it is necessary to extract the lower internal equipment from the vessel and consequently reinstall it after the operation. Both the extraction operation and the operation of reinstalling the lower internal equipment present considerable risks and increase the time required to carry out the maintenance of the reactor at shutdown.
Applicant's French Patent Application No. 2,573,236 relates to a device for cleaning the guide tubes of a nuclear reactor, which can be used without dismantling the lower internal equipment.
Such a device comprises, in particular, a rigid tube of small diameter and of great length, called a needle, which is introduced into the upper end of the sleeve of the tube to be cleaned, after it has passed through the lower internal equipment, by way of the corresponding guide conduit.
Such a device, which must be maneuvered by remote control, is relatively complex and must include monitoring and display means making it possible to ensure centering and perfect positioning of the end of the needle in the sleeve.
Furthermore, the time devoted to installing the device increases proportionately the time devoted to the cleaning of the guide tubes. In nuclear reactors in service at the present time, there are fifty guide tubes passing through the rounded bottom of the vessel by way of fifty sleeves distributed uniformly over this rounded bottom, and the repetition of the installation operations risks greatly increasing the total time devoted to the cleaning of the guide tubes.
In all cases, the water laden with radioactive particles which is recovered at the outlet of the sleeve is sucked up by a pumping device and conveyed to a filtration and decontamination apparatus.